This invention relates to a tool for extracting screws with damaged screw heads. More specifically, this invention relates to a single-piece tool adapted to be installed in the chuck of a counter-clockwise rotating drill to extract screws with damaged screw heads of the type with symmetrical recesses, such as Phillips or Robinson type screw heads.
The prior art discloses a number of threaded devices for removing broken bolts or studs. For instance, U.S. Pat. No. 1,105,535, which issued Jul. 28, 1914 to J. O. Roberts, discloses merely a steeply pitched left-hand auger bit adapted to be inserted into a hole separately drilled in the broken bolt. The bit is turned counter-clockwise to remove the broken bolt.
U.S. Pat. No. 2,062,383, which issued Dec. 1, 1936 to M. A. West discloses a double-ended tool having threads of opposite hand on its respective ends. The reaming and removing end has a beveled tip and the driving end has a flat tip. Both ends operate in the recess of a special screw.
U.S. Pat. No. 2,281,164 which issued Apr. 28, 1942 to H. S. Maling discloses a right-hand threaded sleeve adapted to be screwed into the hole of the broken bolt; the sleeve having a left-hand threaded bore into which a threaded mandrel is screwed, the mandrel having teeth or nibs on its bottom to dig into the bolt remnant so that the remnant is removed as the sleeve is unscrewed.
A screw extractor in the Berendzen U.S. Pat. No. 4,078,458 which issued Mar. 14, 1978 discloses a drill on the end of a tapered threaded cone whereby a hole can be drilled in the remnant and, drilling farther, the cone can engage the margins of the hole to spin the remnant out of its threaded hole.
A number of extractor patents have issued to Eli Polonsky in which a left-hand drill bit is provided with a gripping fixture on its stem, and adapted to bite into the hole made by the drill so that the remnant may be removed in a single operation as the device is turned counter-clockwise. The Polonsky patents are U.S. Pat. No. 4,604,917, which issued Aug. 22, 1986; U.S. Pat. No. 4,777,850, which issued Oct. 28, 1928; and U.S. Pat. No. 5,032,487, which issued Jul. 16, 1991.
U.S. Pat. No. 5,251,516 and U.S. Design Pat. D 340,184, both issued Oct. 12, 1993 in the name of Donald Desaulniers and assigned to Alden Corporation, disclose tools for extracting broken bolts and the like. One end of the tool drills a cylindrical hole in the remnant, and the other end has a gently tapered extraction section with a left-hand thread for biting into the side of the hole thus drilled to enable removing the remnant.
Most of the previous extractor techniques involve first drilling a hole into a remnant in which the screw head is broken off or missing, and then inserting some form of extractor which engages the drilled hole thereby allowing the remnant to be rotated out of its bore. However in many cases, the screw head remains attached to the screw proper, but the screw head is damaged when attempting to remove the screw. This problem is particularly acute when a screw head has a symmetrical recess adapted to mate with a special tool, which damages the screw head while attempting to remove the screw, so that it can no longer obtain a purchase on the screw head. In such a case the tool will merely turn in the recess and slip from one position to the next without generating enough torque to remove the screw. Typical of this problem are Phillips head and Robinson head screws, which become damaged and are difficult or impossible to remove with the regular tools designed for this purpose.
Accordingly, one object of the present invention is to provide an improved tool for removing screws or bolts with damaged screw heads.
Another object of the invention is to provide an improved single-piece tool adapted to be installed in the chuck of a counter-clockwise rotating drill to extract screws with damaged screw heads of the type with symmetrical recesses, such as Phillips or Robinson type screw heads.
Briefly stated, the invention comprises a tool for extracting screws with damaged screw heads of the type with a symmetrical center recess defining circumferentially spaced recess extensions having a major diameter, alternating with resistive sections having a minor diameter. The tool has a central section with flat longitudinal surfaces adapted to be engaged by a drill chuck, a left-hand recess-forming section at one end of the central section, the recess-forming section having a forming distal end with a first portion arranged to make a cylindrical drilled hole with a diameter intermediate the major and minor diameters into the resistive sections and a second flared portion curving away from the axis of rotation so as to form the resistive sections into a flared shape, which is designed to limit the depth of the cylindrical hole so that it does not extend from the screw head into the screw proper. A screw-removing section is provided at the other end of the central section, the screw-removing section having a tapered portion extending toward its distal end and being formed with left hand threads on the exterior of the tapered portion, the tapered portion being at an angle such that the tapered portion threads will engage the flared walls of the resistive sections as the tool is inserted into the recess. Preferably the tapered portion further includes at least one longitudinal gash extending along the tapered portion and interrupting the threads, so as to facilitate engagement of the threads to enhance the screw-removing process. In its preferred form, the tapered portion is frusto-conical with two diametrically opposed gashes skewed slightly with the axis of rotation.